Because the brain is REAL byproduct of extremely complex network of highly organized cells... consisting of molecular arrangement of REAL chemical elements... forged deep within the inferno cores of REAL massive stars... created by gravitational contraction of REAL hydrogen... in none other than our REAL UNIVERSE!

Therefore if "God" exists "in our brain"... then you are technically insinuating that the "conception of God" is logically linked with the REAL physical universe via that vast chemical network we call a "brain".